The subject exemplary embodiments relate generally to alignment of images onto media produced by print or marking engines included as part of an integrated printing system or integrated parallel printing system. A method is provided for aligning images produced by one or more print engines, such as color and black print engines, which permits accurate and objective determination of the relative misalignment of images from one print engine to another print engine. The present exemplary embodiments relate to media (i.e. document or paper) handling systems and systems for printing thereon and is especially applicable for a printing system comprising a plurality of associated marking engines.
Printing systems including a plurality of marking engines are known and have been generally referred to as tandem engine printers or cluster printing systems. See U.S. Pat. No. 5,568,246. It is typical that different sheets within a document will be produced by different marking engines. Such systems especially facilitate expeditious duplex printing (both sides of a document are printed) with the first side of a document being printed by one of the marking engines and the other side of the document being printed by another so that parallel printing of sequential documents can occur. The process path for the document usually requires an inversion of the document (the leading edge is reversed to become the trailing edge) to facilitate printing on the back side of the document. Inverter systems are well known and essentially comprise an arrangement of nip wheels or rollers which receive the document by extracting it from a main process path, then direct it back on to the process path after a 180° flip so that what had been the trailing edge of the document now leaves the inverter as the leading edge along the main process path. Inverters are thus fairly simple in their functional result; however, complexities occur as the printing system is required to handle different sizes and types of documents and where the marking engines themselves are arranged in a parallel printing system to effect different types of printing, e.g., black only printing versus color or custom color printing.
Within a print engine, a document is typically carefully aligned to the print engine's image formation apparatus. The resultant image on paper (IOP) registration performance of each print engine can contribute to the overall appearance of a document. The geometric parameters that are typically specified for each engine are process, cross process, skew alignment, and magnification. It is therefore important that the system is able to correct for any mean shifts between marking engines for any of these parameters, since in general more than one print engine may contribute printed pages to a single finished document. In an integrated parallel printing system comprised of more than two print engines, providing IOP registration consistency can be complex.
The adjustment of the print engines to desired positions for accurate printing can be referred to as image on paper registration. A test print, to be described in more detail below, can contain alignment targets that show each engine's IOP registration relative to an arbitrary base engine. Periodic monitoring and adjusting of IOP registration is desired as the number of marking engines increases or decreases, and/or when the marking engines become misaligned, for example, due to drift. Adjustment mechanisms and controls which maintain IOP registration between multiple printing engines are highly desired.